Thermal presses often form a web of thermoplastic material into yogurt cups, drinking cups, and other types of plastic articles. FIG. 1 is a schematic illustration of a system for forming a continuous web of a thermoplastic material and fabricating finished articles from the web. At the beginning of the process, finely divided pieces of the thermoplastic material are transferred from a bin 20 through an extruder 22, a polishing roll assembly 24, and a rubber roll assembly 25 to transform the finely divided pieces into a continuous web of the thermoplastic material. The web, for example, is generally a continuous sheet having a width of approximately 2-6 feet and a thickness of approximately 0.005 to 0.500 inches. The web moves from the roll assemblies 24 and 25 to a heat tunnel 26 that heats the web to a point at which the material can be physically deformed and still retain a desired shape. The web then moves from the heat tunnel 26 to a thermal press 28.
The thermal press 28 generally uses two complementary molds, such as a plug and a die, to form a section of the web into the finished article. A typical thermal press has a plug platen, a platen drive assembly to raise and lower the plug platen, and a plug assembly attached to the plug platen. The plug assembly typically includes a separate final-drive motor coupled to the plug to move the plug along a drive axis independently from the movement of the platen, and the plug assembly may also include a press plate having an opening aligned with the plug to allow the plug to pass through the press plate. The thermal press also generally includes a separate die platen coupled to the platen drive assembly.
The thermal press 28 drives the plug into the die to form the finished article from the web. In operation, the plug and die assemblies are initially positioned on opposite sides of the web. The platen drive assembly drives the plug platen and die platen toward one another until the press plates press against opposing sides of the web. The final-drive mechanism then drives the plug through the opening in the press plate and into the web. As the plug travels into the die, the web stretches to conform to the shape of the plug and die. A vacuum is also typically drawn in the die and pressurized "form air" is typically blown from the plug to more accurately shape a portion of the web to conform to the shape of the plug and die. After the web is formed into the finished article, a positive air pressure passes through the plug to separate the finished article from the plug. The finished article is then cooled to a point at which it will retain its shape. In a typical application, several plugs are coupled to the final-drive motor and several dies are aligned with the plugs such that a plurality of plugs simultaneously pass into a corresponding plurality of dies to form a plurality of finished articles.
One manufacturing concern related to thermal presses is that the rim of the finished articles may not be formed accurately. The rim of a finished article is formed by the press plates as the platen drive assembly presses the press plates against opposing sides of the web. Conventional thermal presses, however, may not consistently apply the necessary pressure to shape the thermoplastic web in the rim region of the finished articles. As a result, one problem with conventional thermal presses is that they may not consistently produce rims having the desired wall thickness or shape.
Another manufacturing concern of fabricating articles from thermoplastic materials is that the walls of the finished articles may not be formed accurately. For example, the wall thickness of the finished articles may not be uniform or within a desired range. The wall thickness of a finished article is a function of many factors, including the consistency of the vacuum drawn in the dies and the form air from the plugs. If the vacuum in a die or the pressure of the form air is not sufficient, the wall thickness or the shape of the finished article may not accurately conform to the shape of the die. Many conventional thermal presses may not provide a consistent vacuum in the dies or sufficient form air pressure from the plugs because leaks may form between the press plates and the web at the rim of a finished article. Such leaking at the rim is particularly problematic when the press plates do not apply sufficient pressure against the web. Therefore, many conventional thermal presses may not accurately form the walls and the rims of the finished articles.